This invention relates to a fuel injection device, and more specifically to a fuel injection nozzle and holder assembly of the type incorporating a spring-loaded needle valve for the delivery of metered charges of fuel under pressure into a combustion chamber of, typically, a diesel engine.
A typical conventional diesel fuel injection nozzle and holder assembly of the type under consideration, for use with a jerk pump, has a tubular nozzle holder closed at one end by a cap having a fuel inlet and at the other end by a nozzle having spray holes. The nozzle holder defines a storage chamber for temporarily storing each incoming charge of pressurized diesel fuel. Mounted in the storage chamber are a check valve and a needle valve which are biased by a compression spring in opposite directions. The check valve, sprung against the nozzle holder cap, allows communication between the fuel inlet and the storage chamber only during the delivery of each pressurized fuel charge from the jerk pump. The needle valve, on the other hand, is spring loaded into abutment against a valve seat at the tip of the nozzle, normally holding the spray holes closed.
As the check valve blocks communication between fuel inlet and storage chamber upon admission of each required amount of pressurized fuel into the latter, the fuel pressure in the storage chamber unseats the needle valve against the bias of the compression spring. With the spray holes thus uncovered, the fuel is sprayed into the combustion chamber of a diesel engine cylinder, therein to be ignited by the high temperature air at the end of the compression stroke.
This well known type of fuel injection nozzle and holder assembly has problems arising from the fact that the force of the single compression spring determines the fuel pressure needed to unseat the needle valve. For engine operation under light load the spring force resisting the unseating of the needle valve should be sufficiently small to allow the valve to uncover the spray holes under low fuel pressure. However, such small spring force adversely affects engine performance when it is heavily loaded. Since then the fuel pressure in the storage chamber is higher, the needle valve will rapidly rise away from the valve seat upon discommunication of the storage chamber from the fuel inlet by the check valve. The fuel will then be injected abruptly and at unduly high peak pressure into the combustion chamber, causing a sudden pressure rise therein due to combustion and resulting in great noise production and exhaust emission.
These difficulties during engine operation under heavy load can be obviated, of course, by making the force of the compression spring greater. But then the spring force will be too great for proper fuel injection at low pressure during engine operation under light load. Thus the conventional fuel injection nozzle and holder assembly with the single compression spring is unable to meet the contradictory requirements imposed thereon during engine operation under the different conditions.